Clinical magnetic resonance spectroscopy (MRS), both single-voxel MRS and multi-voxel magnetic resonance spectroscopic imaging (MRSI), requires the spatial localization of a volume of interest. The problem is to spatially localize a volume from which to acquire a MRS spectrum that only includes the volume of interest (VOI)—a brain tumor, prostate, or an entire brain, for example—and excludes regions outside the VOI. The overall goal is to produce clinically-beneficial MR spectra that accurately and exclusively represent the VOI.
This is typically accomplished by methods such as Selective Excitation, Outer Volume Suppression/Presaturation, or some combination of these. For selective excitation, conventional methods (pulse sequences) include Point Resolved Spectroscopy (PRESS, see U.S. Pat. No. 4,480,228) and Stimulated Echo Acquisition Mode (STEAM). These pulse sequences excite regions of space (voxels) that are shaped as right parallelepiped (rectangular/cuboidal) volumes.
Outer Volume Suppression (OVS) techniques involve spoiling (zeroing) the magnetization outside the VOI. As a result, these spoiled regions, in theory, do not contribute to the MRS spectrum. OVS is accomplished using successive slice-selective, radio-frequency (rf) pulses and strong gradient pulses to suppress the signal from selected slices. Various techniques for OVS have been developed. However, due to the poor spatial selectivity of OVS rf pulses, these techniques are not suited for use as a primary localization technique. In spite of this limitation, these techniques have been successfully applied in concert with selective excitation techniques to shape the excitation volume. For example, as shown in J. H. Duyn, et al, Radiology 188 (1993) 277-282 (Multi-section proton MR spectroscopic imaging of the brain), a clinical MRS application which employs OVS and selective excitation is MRSI of the brain, in which OVS rf pulses are manually positioned to shape the localized VOI such that the signal from subcutaneous lipid layers are suppressed.
More recently, as shown in T. C. Tran, et al, MRM 43 (2000) 23-33 (Very selective suppression pulses for clinical MRSI studies of brain and prostate cancer), Very Selective Saturation (VSS) rf pulses have been introduced as an improvement to previously-used OVS spatial saturation pulses. The VSS rf pulses have spatial edge profiles that are about an order of magnitude sharper than traditional OVS rf pulses as described in U.S. Pat. No. 6,137,290 (Hurd et al) issued 24 Oct. 2000 entitled “Magnetic resonant spectroscopic imaging having reduced chemicals shift error”.). The application of VSS rf pulses has been successfully demonstrated for clinical MRSI of the brain and prostate. In these cases, the VSS rf pulses were used to suppress signal from the subcutaneous and periprostatic lipid layers for the brain and prostate, respectively, and to significantly reduce the chemical shift displacement effect.
With the above techniques, defining the size and location of excitation voxels (for selective excitation) and/or spatial saturation slices (for outer volume suppression) is done manually using a computer graphical-user interface. Using a computer mouse and keyboard, the scanner operator defines a rectangular box, which represents the excitation cuboidal voxel, and lines, which represent spatial saturation slices (slice volumes in which the magnetization is spoiled by spatial saturation pulses). For analyzing brain tumors using single-voxel MRS, the standard practice is to define a relatively small cuboidal region (the excitation voxel) somewhere entirely within the tumor. Using MRSI, the standard practice is define a relatively large voxel which encompasses the VOI, e.g., brain or prostate, and apply spatial saturation pulses to suppress signal from undesired (eg adipose) tissue that is adjacent to, or within the voxel.
For single-voxel MRS of brain tumors, in which a small voxel is typically defined entirely within the tumor, a limitation is that there will be regions of the tumor not sampled. Thus, since brain tumors are heterogeneous, the resultant spectrum could vary depending on where the operator of the scanner places the voxel inside the tumor. This introduces subjectivity into the results. Furthermore, if part of the cuboidal voxel happens to contain regions from outside of the VOI, then part of the total signal will be contaminated by unwanted signal from outside the VOI. To suppress unwanted signal from outside the VOI, spatial saturation slices, as produced, for example, by VSS pulses, could be manually positioned via a computer interface, but it can be difficult and time-consuming to accurately and optimally position the spatial saturation slices in 3D, especially while only viewing 2D image slices, which is standard for image-viewing with commercial MRI scanners. Another problem with using a relatively small voxel is that more time is needed to acquire a spectrum with acceptable signal-to-noise, as compared to using a larger voxel.
For MRSI, manually placing spatial saturation slices, via a computer interface, suffers from a similar limitation as with single-voxel MRS. Frequently with MRSI measurements there are also regions around the VOI which need to be excluded (“volumes of exclusion,” VOE), for example, subcutaneous lipid layers or lipid around the prostate. It is difficult and time-consuming to manually locate excitation voxels and spatial saturation slices that optimally excludes/suppresses the VOE from contributing to the spectral signal from the VOI.